I think it originates with the disconnect between people and from where their food comes, which was cultivated as industrial agriculture grew. There are some who don't think it's right for humans to even keep animals in captivity at all -- and yes, this is an extreme far end of just one spectrum of "why am I vegetarian/vegan?" which I call the "animal compassion" basis. For these people, it's primarily about not wanting to harm (or sponsor others' harm of) animals, and eschewing foods and products made with their bodies or their products. As so much of our food production now seems to go on behind closed doors, all that's needed to sway more people to this side is the appearance of undercover videos and images of the worst examples of industrial animal agriculture -- since they have no other accurate information presented about how their food is produced, it is assumed that these examples are the norm. To protest, they stop buying products.
But there's another spectrum from which many vegetarians/vegans make their decisions, and that is the one which propels me -- environmental responsibility. Simply put, meat and animal products are far too abundant in the American diet as a result of industrial animal agriculture, with negative consequences for the humans who eat them in too-high amounts and for the environment by way of pollution and squandering of resources. Meat should not be cheap -- until the mid 20th Century, it was something for special occasions, and extended in normal-fare recipes more as flavoring than as featured components. This was the result of animals being raised outdoors by more people on smaller farms. Total production was lower than it is today -- with fewer but much larger farms raising concentrated numbers of animals in largely indoor settings to meet current demand for cheap and abundant meat. The result of concentrating large numbers of animals into fewer farms is toxic buildups of manure and urine, polluting groundwater and local rivers and streams. And the other component of "environmental vegetarianism/veganism" is simple energy economics -- it takes a considerable amount more water and fossil fuels to produce animal-protein calories than it does for the equivalent in plant-protein calories.
I'm not vegetarian/vegan in the classical definition -- I do eat meat, eggs, and dairy. But the meat I eat is ordered directly from the small farms who produce it (farm-to-fork), prepared as occasional fare and cherished when I eat it, because I pay the true-cost of responsible animal agriculture via the higher prices of this kind of meat. For as much as I can with the rest of my diet, I seek vegan (or at least vegetarian) options, and make as much of my own food as possible. I modified my diet as a result of reading
America's Food: What You Don't Know About What You Eat by Harvey Blatt. It's not an emotionally-driven propaganda piece. It reads as a science textbook, written by a geologist, heavily referenced with valid sources and stats. He lays it out there about conventional food production, its effects, and less energy-draining and less polluting alternatives. After reading that, I sought out local food as much as I could (sponsoring farmers directly with my money!), and especially for animal products. Otherwise, I try to eat as low on the food chain as possible.
Maybe someone who is a vegan can pop in and explain it to me.
